Leakage flows, i.e. fluid flowing through the gap between the blade tips and the shroud or casing of a turbine or compressor, are responsible for a significant percentage of overall losses in gas turbines and can also locally increase the heat transfer. Due to different expansion coefficients and heating rates, the tip clearance is not constant, but changes during the turbine start-up and shutdown. Also, wear and tear increases the tip clearance slowly over the time. The turbine has to be designed such that the blades do not touch the turbine shroud or casing under normal operating conditions because it would lead to excessive wear or even damage.
Given real time tip clearance data, future turbines might be able to actively control the tip clearance by adjusting the amount of cooling air to the blades or shroud. Monitoring the tip clearance can also provide valuable information about the condition of the stage for maintenance. Current tip-clearance probes are of inductive or capacitive type such as described in U.S. Pat. Nos. B1-6,437,583, 6,114,862, B1-6,320,394 or B1-6,362,633 having an accuracy of typically 5%. This is sufficient where the probe can be mounted flush with the turbine casing, and the absolute errors can be kept small. The maximum operating temperature of the sensors is near the Curie point of the rare earth magnets used, which is well below the temperature in the first turbine stages. The high temperatures also make it impossible to provide optical access for the optical probes flush with the shroud. When mounting the probes in a protective recess within the shroud, then the relative measurement errors become to large in absolute terms.
U.S. Pat. No. 5,572,039 discloses an apparatus which enables the clearance between seal fins on rotating blades and adjacent fixed structure to be observed during rotation and comprises refracting prisms on the fixed structure arranged so as to straddle sealing fins on the blades. The stage of blades on their associated disc are moved toward the fixed structure and light, which is refracted through the prisms, is obscured by the fins. In one embodiment, the ratio of obscured to unobscured light is utilized to generate electrical signals, which are then manipulated so as to indicated the magnitude of the clearance.
U.S. Pat. No. 5,739,524 concerns a dynamic sensor for sensing the distance and the position of a surface moving past said sensor.